1. Field of the Invention
The present invention relates to an amplifier composed mostly of transistors.
2. Description of the Prior Art
FIG. 3 shows an example of a conventional differential amplifier. This amplifier has a pair of transistors T1 and T2 whose emitters are both connected to a constant current source 35 and whose bases are respectively connected to a first and a second input terminal 31 and 32. The collectors of the transistors T1 and T2 are respectively connected to transistors T3 and T4 that form a current mirror circuit, and the output from these transistors T3 and T4 is delivered through a transistor T5 to an output terminal 33. Numeral 34 represents a power line for the supply of a direct-current voltage Vcc, and numeral 36 represents a constant current source that is connected to the emitter of the transistor T5. Thus, an input signal applied between the input terminals 31 and 32 is amplified and then delivered to the output terminal.
Inconveniently, however, this amplifier cannot be kept in operation unless the base currents of the transistors T1 and T2 are kept supplied via the input terminals 31 and 32. Moreover, as the constant current I is increased, the base currents supplied need to be increased accordingly. Furthermore, in this amplifier, if there is an error between the base currents supplied via the first and second input terminals 31 and 32, it causes an undesirable offset voltage to appear at the output.